U.S. Pat. No. 8,406,932, Hollaway discloses the use of a single temperature sensor in the body of the heater to monitor water flow conditions through the heater and to also measure water temperature in the spa. Water flow rates are estimated by the amount of time it takes for the heater to change from one temperature to another, with the pump running normally. The rate of change is, therefore, more important than the actual temperatures. Each time the pump and heater are activated due to an apparent need for heat, based on the water temperature inside the heater, or the length of time since the last heat cycle, the pump will be turned on long enough to compare the real water temperature with the estimated water temperature. Any difference will be recorded and applied as an offset to the next activation. New offset errors will recorded with future activations, adapting the process to changes in ambient conditions.